EEStor and Zenn cannae break the laws of physics

18 March 2007

We keep hearing odd things about US startup the Zenn car company and its exclusive deal with ultrasecretive ultracapacitor startup EEStor [PDF]. An ultracapicitor, for the uninitiated, is like a big battery but it stores energy as a static electric field, rather than through a chemical process. You can think of it as a bag into which you can shove electrons, keep them there for as long as you want (given a little leakage), and get them out again as rapidly as you like (with much less waste-heat-generating internal resistance than a battery).

According to more than one enthusiastic report, the combination of these two inventive companies will result in a car that can travel 200-300 miles on just a five-minute charge. Real soon now.

Well, we are here to tell you that, whatever technology lurks inside the eventual EEStor-powered Zenn, it will not be charging itself up in five minutes.

The problem lies with the nature of electricity.

Let’s be ultra-generous and assume that the EEStor and Zenn combination will be lots more efficient than the G-Wiz and will squeeze 200 miles out of the same amount of stored energy that takes the G-Wiz just 40 odd miles: 10kwh. This is not really feasible, but it is being very kind to the two plucky startups. It might just be possible if you drive veeeeerrrrrrry sloooooooooowly.

Anyway, to get 10,000 watt-hours into the battery in five minutes you need to feed in at a rate of 120,000 watts. This is just basic maths. You can imagine this quite simply: it’s like plugging in 120 single-bar electric fires and switching them on at once. As you might imagine, this will likely blow the odd fuse in your house.

Put another way, to supply this from a domestic UK 240 volt supply, you will need to charge at 500 amps. The maximum you can get out of one three pin socket is 13 amps, so you will need to plug your Zenn into about 40 sockets at the same time to avoid blowing any plug fuses.

Assuming you have 40 spare sockets, and the patience to plug them in, you’ll immediately blow the main fuse. Domestic electricity supplies just aren’t built to supply 500 amps.

So will you instead charge your Zenn at a sort of electric petrol station? No. First, there are none. Second, a 500-amp cable would need to be roughly 40 times as heavy and stiff as a domestic kettle’s cord. Unless you pump iron every day, you’ll find it difficult to plug in.

No, it’s much better to charge an electric car over a period of hours, during the part of the day when it’s not moving (given that nobody – barring a few madmen at Le Mans – drives for 24 hours a day).

Instead of charging over five minutes, charge for 40 times as long – at least 3 hours and 20 minutes – and you can use a normal three pin plug.

That’s the future of electric cars, no matter what. Unless you use a fuel plus generator or fuel cell to make electricity on the spot, electric cars are going to take hours to charge. No breakthrough in battery-style storage is ever going to change that. It’s just maths.